The present invention relates to a method of transmitting digital signals by radio over a predetermined frequency band, a repeater station for implementing the method, and a radiocommunications network using said method.
Radiocommunications networks are known that comprise a series of telephone exchange stations connected to a series of fixed base stations distributed over the territory to be covered by the communications network, each fixed base station including a radiotelephone interface member, a transmitter, and a receiver for communicating with mobile stations over a radio link.
Establishing a base station involves a certain number of constraints, in particular the presence of a mast carrying the antennas that enable radio links to be established with mobile stations, together with premises for housing the base. Such establishment also requires telephone cables to be laid for the purpose of connecting the base station to the telephone exchange stations.
Furthermore, attempts are currently being made to reduce the power and the range of mobile stations so as to limit the volume and the cost thereof. The range of mobile stations is also reduced by using higher frequencies for the radio links between mobile stations and fixed base stations. It is therefore necessary to increase the number of base stations, which solution becomes prohibitively expensive in zones of low communications density (rural zones).
One solution to that problem consists in using one or more repeater stations for one or more of the fixed base stations (see EP-A-0 415 810).
In general, the repeater stations merely comprise amplifiers connected between reception antennas and transmission antennas. Because the same radio-frequency is used for transmission and for reception, repeater stations of the above structure can be used only when the conditions of use of the repeater station provide sufficient isolation between the transmission antennas and the reception antennas. This criterion is satisfied adequately in certain cases, in particular when the zone covered by transmission from the repeater station has special radio propagation characteristics, for example in the presence of a tunnel or of some other underground works, or when the transmission antennas are devices providing localized radiation, e.g. lossy cables.
This solution is therefore unsuitable for satisfying the need mentioned above.
It is also possible to use repeater stations that implement frequency transposition. Thus, in certain applications, repeater stations are known where transposition is performed in a frequency band reserved for this type of transmission, e.g. the 38 GHz band (see EP-A-0 415 810).
However, it is often not possible to use a frequency band reserved for this purpose, in particular because of regulations. In theory, it should be possible to use available frequencies in the band reserved for communication with mobile stations. Nevertheless, that solution suffers from the major drawback of disturbing the operation of the mobile stations which may interpret a radio link established in this way between the fixed base station and the repeater stations as a radio signal intended for mobile stations.